CVE-2026-55557

### Impact `browser_download` wrote a fetched file to `join(save_dir, filename)` with no validation of `save_dir`, and `browser_save_state` / `browser_load_state` honored an explicit `path` unchanged. The MCP caller controls these arguments (a malicious MCP client, or an autonomous agent steered by indirect prompt injection on a visited page), so an attacker could supply an arbitrary `save_dir` (or state `path`) together with a URL whose response body became the file contents, writing attacker-controlled bytes to any path the process can reach (for example `~/.bashrc`, an autostart entry, or a cron file). That is an arbitrary file write that can lead to host code execution. The `force_fetch` fallback additionally used a raw `fetch()` that bypassed the `BROWSE_MCP_ALLOWED_ORIGINS` origin fence. Estimated severity: CVSS 3.1 around 7.8 (High) for the local / agent-mediated case. ### Patches Fixed in 0.8.2. `save_dir` is confined under the download root (`~/.browse-mcp/downloads`) and the explicit state `path` under `~/.browse-mcp/state`; absolute paths and `..` escapes are rejected, and download filenames are reduced to a bare basename. `force_fetch` now also honors the origin fence. Data roots remain relocatable via `BROWSE_MCP_HOME`. Upgrade to browse-mcp 0.8.2. ### Workarounds Restrict the exposed tools with `BROWSE_MCP_TOOLS` to a set that excludes `browser_download`, `browser_save_state`, and `browser_load_state` (for example the hardened recipe in SECURITY.md). Note that the allowlist hides the tools from an agent but does not stop a malicious MCP client from calling them by name, so upgrading is the real fix. - Reported privately by novice-22.